Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu
Congratulations Bob Leiby for correctly identifying the damage as mechanical and for being selected to be entered into the end of season raffle for $100 not once but five times. Everyone else who guessed correctly will also have their name entered into the raffle. Click on the Guess the Pest logo to participate in this week’s Guess the Pest challenge!
Guess the Pest Week #19 Answer: Hole Puncher
By David Owens, Extension Entomologist
Photo by Joe Deidesheimer, defoliator is Kevin Troyer
This week’s guess the pest was a bit of a trick question, the answer is hole puncher operated by a hard-working student. Soybean canopy defoliation can be a little tricky to estimate, defoliation often appears more severe than it really is because our eyes focus on differences. We are simulating bean leaf beetle feeding injury to R-2 stage soybean by removing approximately 25% of the foliage canopy-wide.
Although this looks really severe, soybeans can compensate for this level of defoliation. Our threshold for defoliation at this soybean stage is 20% CANOPY and FIELD wide. Our most common defoliators right now feed primarily in the upper canopy. So if 25% of the upper canopy of R-stage soybean is defoliated, but only 5% of the lower canopy, total defoliation could be lower than 15% and the plants will not suffer a yield impact. If there is little to no defoliation in the lower canopy, the upper canopy can take a severe beating before canopy-wide defoliation hits 20%. We may start seeing soybean looper later in the season, this species often defoliates from the bottom up.
Vegetative stage soybean can compensate even greater defoliation. Recent work out of Mississippi indicates that 66% of the canopy of VEGETATIVE beans can be lost without a significant yield loss. In the Mississippi study they also defoliated beans during vegetative growth, at R3, and constantly during the season to simulate the impact of multiple sub-threshold ‘dingers’, and found that a constant 17% defoliation did not significantly reduce yields.
Two other important factors that reduce soybean’s compensatory ability are drought and planting date. Late planted beans have less time to recover from severe defoliation and may (but not always) loose yield. Drought stress may also reduce this compensatory ability. The Mississippi defoliation experiments involved a small army of students around the clock picking leaves off of over 100 10-ft plots.